1. Field of the Invention
The present invention relates to a detection and ranging apparatus and detection and ranging method having a function for estimating the direction of arrival of a signal by using a sensor array and a direction-of-arrival estimation method.
2. Description of the Related Art
As an example of such a detection and ranging apparatus, consider a radar apparatus capable of estimating all three quantities, i.e., relative distance, relative velocity, and angular direction of a target. In a radar, a sensor refers to an antenna. FM-CW radar is a type of radar that projects forward a transmitter wave frequency-modulated by a triangular wave or the like, and receives a reflected wave from a target, mixes it in a mixer with a portion of the transmitted signal to obtain a baseband signal containing information on the target, and calculates the distance and relative velocity of the target from the baseband signal, and this type of radar is commercially implemented as automotive radar, etc.
One possible method for determining the direction in which the target is located is to use an array antenna having a plurality of antenna elements to receive the reflected wave from the target, and to determine the direction of the target by applying a direction-of-arrival estimation method such as a known beam former method to the received signal.
In a direction-of-arrival estimation method using an array antenna, for example, in the beam former method that scans the main lobe of the array antenna in a given direction and determines the direction where the output power is the greatest as being the direction of arrival, the beam width of the main lobe determines the angular resolution, and therefore, if it is desired to increase the resolution so that the directions of many targets can be determined, the aperture length of the array must be increased by increasing the number of antenna elements. The same can be said of the minimum norm (Min-Norm) method that determines the direction of arrival from the eigenvalue and eigenvector of the correlation matrix of the array's received signal, and its extended algorithms such as MUSIC (MUltiple SIgnal Classification) and ESPRIT (Estimation of Signal Parameters via Rotational Invariance Techniques), i.e. in these methods also, since the dimension of the correlation matrix, i.e., the number of antenna elements, determines the number of targets that can be detected, the number of antenna elements must be increased in order to be able to determine the directions of many targets.
However, in the case of a radar apparatus such as an automotive radar where severe constraints are imposed on the mounting dimensions of the antenna, it has been difficult to increase the number of antenna elements without compromising reception power.
JP 2006-98181A and JP 2000-155171A each propose a method that enlarges the effective aperture by using a plurality of transmitting antennas.
In this case, for each received reflected wave, the transmitting antenna that transmitted the original radiowave must be identified. In the above patent documents, each one of the plurality of transmitting antennas is selected for use in time division fashion by using a switch so that the transmitting antenna can be identified for each received wave. Further, on the receiver side each one of the plurality of receiving antennas is selected in time division fashion by using a switch, in order to reduce the cost by reducing the amount of RF circuitry.
However, switching from one antenna to another using a switch involves adverse effects such as degradation in signal and reduction in detection range. Furthermore, since it does not follow that the waves transmitted out from the plurality of transmitting antennas and returned by reflection are received simultaneously by the plurality of receiving antennas, there arises the problem that a time shift and a phase shift occur. This problem can be alleviated by increasing the switching speed, but there is naturally a limit to it.
Further, in JP 2001-237755A, which relates to a different field than the present invention, i.e., to a so-called phase monopulse scheme in which radiowaves transmitted from two antennas at a base station in a mobile communication system are received by one antenna at a mobile station to determine the direction based on the difference between the arriving phases, it is described that the radiowaves are spread using mutually orthogonal spreading codes so that the transmitting antennas that transmitted the respective radiowaves can be discriminated at the receiving end.